They *are* battery-backed RAM, but they're normally factory-preset to include some data the game checks for in order to boot (rather like the copy protection dongles some PC apps use).

The MAME ROM sets include data read from working TimeKeepers to allow the emulated game to pass its copy protection and that data can be programmed into a new TimeKeeper to "un-suicide" the hardware. I don't know the exact procedure because I haven't had to do it myself, but RetroRepair indicates they can be read like a 27C64 in an EPROM reader so I would try just programming it at a 27c64. The data you need should be the file called "m48t58y".